aarch64: Generalise _m rules for SVE intrinsics

aarch64: Generalise _m rules for SVE intrinsics

In SVE there was a simple rule that unary merging (_m) intrinsics
had a separate initial argument to specify the values of inactive
lanes, whereas other merging functions took inactive lanes from
the first operand to the operation.

That rule began to break down in SVE2, and it continues to do
so in SME.  This patch therefore adds a virtual function to
specify whether the separate initial argument is present or not.
The old rule is still the default.

gcc/
	* config/aarch64/aarch64-sve-builtins.h
	(function_shape::has_merge_argument_p): New member function.
	* config/aarch64/aarch64-sve-builtins.cc:
	(function_resolver::check_gp_argument): Use it.
	(function_expander::get_fallback_value): Likewise.
	* config/aarch64/aarch64-sve-builtins-shapes.cc
	(apply_predication): Likewise.
	(unary_convert_narrowt_def::has_merge_argument_p): New function.